Theoretical Physics Course · Mechanics
As everyone knows, physics consists of two main disciplines: experimental physics and theoretical physics. The large number of physical laws we know can be derived from a small number of very general principles. Such derivation, and the establishment of those general principles, call for a distinctive method, and this method defines a particular branch of study—namely, theoretical physics.
Theoretical physics uses mathematical tools and methods to arrive at its own results and conclusions. However, theoretical physics differs fundamentally from mathematics in that it has a direct link to experimental results. This is not to suggest that the most general laws can only be built on experimental data, nor that drawing conclusions from those laws does not also require prior experimental investigations. Without such investigations, one cannot judge which among the many interwoven factors are important or negligible. Once the relative importance of these factors is known, the essential task of theoretical physics is essentially complete. Further application of these equations to specific cases of varying complexity soon becomes a matter of purely mathematical study, forming what we call “mathematical physics.”
The goal of theoretical physics is to establish physical laws, that is, to establish relationships among physical quantities. Determining the specific numerical values of those quantities is generally not the task of theoretical physics, since, for numerical issues, experimental methods are often simpler and do not require labor-intensive calculations. Naturally, if a situation is simple enough, theory can directly compute the numerical values.
It must be emphasized that theoretical physics aims to establish and characterize the relationships between the physical quantities of a given phenomenon. Consequently, one can only devise a proper theory if such relationships truly exist in nature. Yet in many cases, the physical quantities of interest bear no relation to each other at all; in other words, they belong to entirely separate categories in different natural phenomena. Hence, in certain situations, the absence of a dedicated theory does not imply an inability to explain that phenomenon; if the most general laws can yield the same result, there is no necessity for a specialized theory.
Approximate analysis plays a tremendous role in theoretical physics. First, every “exact” law is in reality approximate, because in the vast majority of cases, that approximation offers sufficient accuracy. Second, theoretical physics does not strictly demand absolute accuracy in physical laws. If one defines the scope of a given phenomenon in advance, it suffices for the outcome to meet the required degree of precision. That is why we can still use Newtonian mechanics for analyzing the trajectory of artillery shells, despite knowing it is not absolutely accurate, simply because it is sufficiently precise in that domain, and we turn to relativity only when necessary for higher accuracy.
For this reason, in theoretical physics, there coexist certain theories (often referred to as “classical theories”) that have been shown to be less accurate alongside those that are more exact. They remain useful because, within certain specific ranges of phenomena, they retain their applicability. Any logically complete theory, once verified as valid within a certain accuracy range, does not lose its value. Indeed, partial or approximate results, derived in particular cases, remain embedded in any subsequent, more precise theory. Plainly, this category also includes those still under development or not yet fully coherent; they, too, have significance in the progression of theoretical physics.
Thus, we see that a key process in general physical theory lies in deducing more specific laws from the most general principles, without neglecting the central role of careful consideration of the most important factors. Overlooking those primary factors while relying solely on coarse simplifications can lead to ignoring the true scale or magnitude of the phenomena. In reality, the forms of phenomena themselves are often approximate, and the functional relationships among the physical quantities that describe them are similarly approximations. When studied at higher levels of precision, these relationships may reveal deeper meanings.
Determining the level of approximation at which one examines a phenomenon is exceptionally important in theoretical research. The gravest error is to adopt an extremely precise theory and exhaustively compute every subtle correction, while failing to recognize the broader advantages that a more streamlined or holistic approach might offer.
L. D. Landau
1940
(Note: Landau wrote this preface in 1940, when computational tools were very limited, so numerical experiments remained challenging.)